This working paper from Tideline, with support from the Bill & Melinda Gates Foundation, offers clarity and practical guidance to the prospect of impact investment as a funding source for product development partnerships to complement ongoing financial support from public and philanthropic donors. The research is intended for an audience of global health actors and stakeholders, as well as current and potential investors in the global health market. It showcases three PDPs (FIND, IAVI, and PATH) in the global health R&D space and identifies opportunities for return-seeking investment using each model.